The present invention relates to communication terminal units connected to an ISDN (Integrated Services Digital Network), and more particularly to a communication terminal unit whose response is limited by a user number, a subaddress and ability specification information included in a call set-up signal sent from a calling terminal.
In addition to major conventional networks for communication terminal units including telephone service networks and telex service networks, ISDNs which handle digital information are increasingly gaining in popularity in recent years.
The ISDN is an integrated communication network, in which information of a wide variety of communication units, such as telephones, facsimile machines and teletypewriters, connected to a digital service unit (hereinafter referred to as "DSU") through buses is integrated in a digital format and communication services to each communication device are taken care of on a single common number basis. A plurality of communication units can be connected to the DSU.
A call set-up signal of the ISDN may include, other than a user number, a subaddress and ability specification information.
The subaddress is a kind of extension number. If a subaddress is included in a call set-up signal, only a communication unit to which such subaddress is assigned can respond and operate from among a plurality of communication units connected to a DSU.
In addition to its use as an extension number, various other uses have been proposed for the subaddress. For example, in Published Unexamined Japanese Patent Application No. 22196/1989, a multi-functional communication device, such as a telephone serving also as a facsimile machine, assigns subaddresses to each of its functions, so that the concept of a subaddress can be utilized effectively.
The term "ability specification information" means a condition or conditions requested by a calling terminal for a desired communication. For example, if the target communication terminal is a facsimile machine, the ability specification information for this terminal includes terminal characteristics and protocol attributes such as a G3/G2 function, a G4 function and a telephone function. These conditions are set as options at the calling terminal.
The aforesaid conventional system assigns a subaddress to each function. Under such system, if a large establishment in which a plurality of departments have telephones, facsimile machines and multi-functional communication equipment dedicated to themselves, it is required that each terminal unit be given a subaddress unique to the department and function; e.g., "101" to the telephone function for department A, "102" to the G3/G2 function for department A, "103" to the G4 function for department A and "201" to the telephone function for department B. This has entailed the problem of making the number management complicated.
Also, for communication terminal units shared in common by a plurality of departments, their number management becomes irregular, making the number management further complicated.